Abstract. Pregnancy is an intricately orchestrated process where expanded tolerance to non-self antigens expressed by the developing fetus and host defense against infection are each simultaneously maintained. Reciprocally, catastrophic complications arise when either host defense or tolerance to the fetus is disrupted. A number of human pathogens that includes Listeria, Plasmodium, E. coli, Group B Streptococcus, Salmonella, Chlamydia and cytomegalovirus each have a defined predisposition for prenatal infection that often results in spontaneous abortion or stillbirth. Therefore, unraveling the immune defects that cause these naturally occurring holes in host defense during pregnancy has direct implications for new therapies aimed at boosting immunity against prenatal infection. We have assembled a novel set of transgenic mouse tools that allow the natural heterogeneity between maternal and fetal antigen to be recapitulated, and the precise identification of maternal immune cells with fetal specificity each during pregnancy. Our overall hypothesis is that the physiological expansion of immune suppressive maternal regulatory T cells (Tregs) required for sustaining tolerance to the developing fetus compromises host defense against pathogens that cause prenatal infection. This is based on our recently published initial studies establishing susceptibility to prenatal Listeria infection is dictated by expanded maternal Tregs. Therefore, with the newfound heterogeneity and functional specialization among regulatory T cells that use unique cell-intrinsic molecules to mediate context specific immune suppression, our first goals are to identify the maternal Treg intrinsic molecules that compromise host defense and dissociate these from other cell intrinsic molecules required for sustaining fetal tolerance during pregnancy. On the other hand, the immune response to infection also has built in ways to override the impacts of Treg suppression that stimulate immune activation and optimal host defense against infection. During pregnancy however, these transient reductions in maternal Treg suppression also fracture tolerance to the developing that can dictate fetal injury or resorption. Accordingly, our secondary goals are to investigate how prenatal infection impacts maternal Treg-mediated fetal tolerance. The first two aims will build upon a productive line of investigation illustrated in our recent publications and initial studies using prenatal Listeria infection to dissect the molecular basis for how maternal Tregs cause infection susceptibility, and to identify the Listeria-specific virulence determinants required for overriding maternal Treg suppression. To establish the broader applicability of these findings, the final aim will investigate if overriding maternal Treg-mediated fetal tolerance also occurs for other pathogens that cause prenatal infection (e.g. Plasmodium, E. coli, Group B Streptococcus, Salmonella, Chlamydia, and cytomegalovirus). The completion of these experiments will unravel how maternal Tregs cause prenatal infection susceptibility, and establish how infection-induced shifts in Treg-mediated fetal tolerance dictates injury to the developing fetus during prenatal infection.